Variable directive coupler



Filed 001;. 24, 1945 llEI-E-r 1 JOHN N. MARSHALL Patented Nov. 28, I950 2,531,777 VARIABLE DIRECTIVE COUPLER John N. Marshall, Washington, D. 0., assignor to Minister of Supply in His Majestys Government of the United Kingdom of Great Britain and Northern Ireland, London, England Application October 24, 1945, Serial No. 624,297

4 Claims.

This invention relates to electric wave coupling means and more particularly to directive electric wave coupling devices.

An object of the invention is to provide means for coupling high frequency power from a transmission line to utilization means for indicating the amount of power passing through the line in a predetermined direction only.

Another object of the invention is to provide means for utilizing a portion of the high frequency power flowing in a desired direction only through a transmission line.

A further object of the invention is to provide a novel'directive electric wave coupling device.

An understanding of the invention may be had by reference to the following description and to the accompanying drawing, in which:

Fig. 1 is a cut-away view of an exemplary embodiment of the invention, and

Fig. 2 is a cross-sectional view of the embodiment shown in Fig. 1, taken upon the plane indicated by the line 2--2.

Referring now to Fig. 1, the invention provides means for coupling a portion of the power flowing in-one direction through a transmission line I to utilization means which comprise a high frequency ammeter I3. In Fig. 1, the invention is arranged so that the ammeter I3 provides a relative indication of the amount of power flowing through line II from a high frequency generator I5 to a load, such as antenna I'I. Power flowing from the antenna toward the genera.- tor I5 produces a negligible current through the ammeter I3. Consequently, the electrical coupling provided by the invention between the line II and the utilization means comprising ammeter I3 may be said to be directive. In the arrangement shown in Fig. l, the invention enables the ammeter I3 to provide an indication of the power output of the high frequency generator I5, while avoiding a false indication due to signal power intercepted by the antenna I7 and transmitted back along the line H toward the high frequency generator I5. The utility of the invention is not, however, limited to the application herein illustrated and described.

The structure of an exemplary form of the invention will now be described with reference to the drawing. The invention comprises a metallic box I9 enclosing a portion of each of two coaxial transmission lines II and 2|. The outer conductors of the lines II, 2| are conduc tively connected to the box I9 at the junctions therewith. Within the box I 9, the inner conductors of lines II, 2| are substantially parallel over a portion of the length of the inner conductor of the line I I. The outer conductors of the lines II, 2| are discontinuous within the box I9, leaving the parallel portions of the inner conductors of lines II, 2| exposed. An electrostatic shield comprising a plurality of conductive wires 23 is positioned perpendicular to the plane of the parallel portions of the inner conductors of lines II, 2|. The wires 23 are conductively connected at each end thereof to the interior surface of the box 2| the same side of the box I9. The exterior portions of the line 2| are joined mechanically by a rigid connection such as a bar 24. The line 2| passes through the sides of the box through bushings 25 which form a sliding fit with the outer conductor of line 2|, thus enabling the distance between the parallel portions of lines I I, 2| to be adjusted by moving the bar 24 in thedesired direction. Spring metal contact fingers 26 (Fig. 2) maintain a sliding conductive connection be-- tween the outer conductor of the line 2| and the interior surface of the box I9.

Consideration will now be given to the electrie cal properties of the invention. In order that no impedance discontinuity be introduced by the removal of the outer conductors of the lines I I, 2| within the box I9, it is necessary to maintain the characteristic impedance Z0 of the lines I I, 2| constant throughout their length. Accord-.-

ingly, the exposed portions of the inner conductors of the lines II, 2| are expanded to a,

The length of the inner conductor of line 2| parallel to the inner conductor of line II is pref erably made 0.5 wavelength, as determined by:

the frequency of the generator I5. The end ofthe line 2| toward the generator I5 is preferably terminated in an impedance 2'! equal to the char acteristic impedance of the line 2|. The end of the line 2| toward the antenna II is terminated by the series combination of the ammeter I3 and an impedance 253 having a series impedance equal to the characteristic impedance of the line 2|.

Power generated by the generator I5 is transmitted to the antenna I! through transmission line I I. The resulting current and voltage in the line II set up magnetic and electric fields which travel along the line H from the generator I5 to the antenna I'I. Within the box It, the trav:

The line 2| is bent within the box Illa so that the line 2| enters and leaves through,

3 eling magnetic field expands and induces two Waves traveling in opposite directions along each elemental length of the inner conductor of the line 2!. The wave traveling in the direction of the antenna I! along the portion of the inner conductor of line 2| parallel to the inner conductor of line I I builds up as the: voltages induced in each elemental length of the inner conductor of the line 21 by the traveling magnetic field add in time phase along the inner conductor of line 2| and is absorbed by impedance 29.,thereby causing an indication on ammeter 1'3. However, the traveling wave proceeding along the inner conductor of the line 2| in the direction of the generator l5 diminishes due to the successive phase shift of the voltages in each eiementar length of the inner conductor of line 2!. The total phase shift along the one-half wavelength section of the inner conductor of line I! is 360 degrees. Consequently, no voltage is applied to the'terminating impedance 2? because the trayeling wave diminishes to zero amplitude in the direction of terminating impedance 2?. For example; consider the situation wherecoupling occurs only at two imaginary points along line H which are separated by a quarter wave length; Two waves'travelling in opposite directions will be'induced at each'of adjacent points'in line 2|. Said adjacent-points are therefore also a quarter wave length apart. Assume a voltage wave travellingin nne H from left-to right; The voltage" induced inline 2! at the left imaginary point and'travelling to the right is'in phase with the inducedvoltage at the right imaginary point travelling iii that direction since the respective induced voltages travel the same distance to reach'i the right imaginary point. However, the induced voltage inline'Zl from the left imaginary point travelling to the left is out of phase with the voltage induced at-the right imaginary point and travelling in the same direction since the erably broader in the specific embodiment shown in Fig. 1 wherein the number of coupling points is infinite.

The directive coupling provided by the inven-' tion will be apparent from a consideration of the condition when antenna l! intercepts power from a wave signal in space. The signal power travels from the antenna iii along the line' H 't0- ward generator i5. Consequently, thepreceding analysislof' 'the operation of the invention is-reversed. That is, the signal power fromthe'antenna I! induces-traveling waves one of which diminishes to-zero in. the direction of the termi nating impedance- 2S; thus producing no indication on ammeter l3. The wave traveling toward impedance 2? builds up and is absorbed by impedance 21.

It is apparent that the directivity of the coupling-provided by the invention may be reversed simply by interchanging the generator and" the" antenna ll.

By" moving bar 27 toward or'away from the box 2 thedistancebetween lines 'll and-2l may: ha adjusted] thereby controlling thenegree of" The frequency band may be broadened r byproviding more coupling points. It is consid-' 4 coupling between the lines H, 2|. Thus, the invention provides an adjustable degree of three-- tive coupling.

Since the ammeter 13 indicates power trans-- mitted along the line I I only in the direction of the load comprising antenna H, the impedance or the antenna I1 should match the characteristic impedance of the line: H, in order to prevent reflection of power along the line in the opposite direction. However, the accuracy of the power indication is relatively independent of a slight impedance mismatch. For example, when the line H is terminated by a load producing 33 percent reflection, the error of the power indication provided by the invention is only 0.5 decibel. The error introduced in conventional current-v or voltage-operated power indicators under the same conditions may be as high as :3 decibels.

In a modified form of the invention, the electrostatic shield comprising conductors 23 is omitted and the' interior of the box I9 is filled with non-conductive material having a dielectric constantconsiderably greater than unity. In

a this modified form the magnetic field is rendered ineffective by the dielectric material and the coupling between the lines H, 2| is produced by means of the electric field, since thedielectric material has, the eiiect of increasing the electric field intensity within the box l9. I

It will be understood that the invention is not limited by the embodiment herein described and illustrated and that the scope of the invention is to be determined from the appended claims.

What is claimed is: V 1. An electric wave directional coupler comprising two transmission lines, means for providing magnetic coupling between two substan tially one-half wave length sections of the lines, electrostatic shielding means positioned between the" lines, and separate impedance means attached to each end of atleastone of said lines] for terminating same in its characteristic impedance. V

2. An electric wave directional coupler com: prising two transmission lines, means for providingmagnetic coupling between two substantially one-half wave length sections of the lines,

electrostatic shielding means positioned between the lines, means for adjusting the degree of magnetic coupling, and separate impedance means attached to each end of at least one of said lines" for terminating same in its characteristic impedance.

3. An electric wave directional coupler comprising two parallel one-half wavelengthcon ductive elements, a conductive surface enclosing;

the conductive elements: coaxial transmission lines terminatlng'the respective conductive elements, the inner conductors of the lines being connected to the conductive elements andthe outer conductors of the lines b'eing connected to the conductive surface, separate impedance meansc'onnected to the remote ends of the trans mission'lines terminating one of'said conductive elements, said impedance means providing a termination of each of said last named lines in its characteristic. impedance, electrostatic shielding" meanspcsitioned between the conductive ele-' ments; and means foradjustingthe distance be tween the conductive elements.

4. In combination, a firstcoaxial transmission" line, a" second coaxial transmission line, a: U=shapedsection1in said second line arranged with its basdmemberparallel to said firsttrans;

5 6 mission line, an electrostatic screen interposed 7 UNITED STATES PATENTS between said first line and the U-shaped section 7 Number Name Date of said second line and forming part of the outer r o conductor of each line for substantially a half gigg g; 5''" S3; wave length thereof. said U-shaped section being 5 9 Gilman 5* 1940 movable with respect to said first line, whereby 3475 Alford Jun' 1941 power flowing in a predetermined direction in said first line will induce a voltage travelling in FOREIGN PATENTS the predetermined directibn in said second line. ,Number Country Date 10 1,545,9ss Great Britain June 18, 1942 OTHER REFERENCES REFERENCES CITED U E ten lliinstrugent fo Dgect giessureFmegt of tlge The following references are of record in the a gs s s g t fi z g g file this Patent: vb1. No. 239, August 1943.

JOHN N. MARSHALL. 

